There is a superconducting cable for transmission and distribution of electric power which uses, as a material, a superconductor having an electrical resistance value of substantially zero under a cryogenic temperature. In order to secure excellent power transmission efficiency of the superconducting cable, it is necessary to stably maintain the superconducting cable in a cryogenic state, and study and development of a cooling system having such a cooling ability are promoted. Note that, in general, a high-temperature superconductor is used as the material of the superconducting cable, and liquid nitrogen is used as a coolant for cooling.
Herein, with reference to FIG. 10, the structure of a conventional superconducting cable cooling system 100 (hereinafter referred to as “a cooling system 100” appropriately) will be briefly described. FIG. 10 is a structural view schematically showing the entire structure of the conventional superconducting cable cooling system 100.
The cooling system 100 has a superconducting cable 1 formed of the high-temperature superconductor as a cooling target, and uses liquid nitrogen as the coolant for cooling the superconducting cable 1. The coolant having cooled the superconducting cable 1 is temporarily stored in a reservoir tank 2. In the reservoir tank 2, the coolant is pressurized to a specific pressure value by a pressurization device 3, and is stored. A controller of the pressurization device 3 which is not shown acquires the pressure value detected by a pressure sensor 4 provided in the reservoir tank 2 and feedback-controls the pressurization device 3 such that the acquired pressure value has a specific value, and the specific pressure value is thereby maintained.
A circulation pump 5 is provided on the downstream side of the reservoir tank 2, and the coolant stored in the reservoir tank 2 is pumped to a refrigerator 6 by the drive of the circulation pump 5, and is cooled. The refrigerator 6 is a GM refrigerator or a Stirling refrigerator. The coolant cooled by the refrigerator 6 is supplied to the superconducting cable 1 again and used for cooling in a superconducting state. A temperature sensor 7 and a flow rate sensor 8 for detecting the temperature and the flow rate of the coolant are provided on the downstream side of the refrigerator 6, and the refrigerator 6 is feedback-controlled such that the temperature of the coolant has a specific value by cooling based on the detected values thereof.
Thus, in the example shown in FIG. 10, a circulation cycle in which the coolant having a temperature increased by cooling the superconducting cable 1 is cooled via a circulation path 9 provided with the reservoir tank 2, the circulation pump 5, and the refrigerator 6, and then supplied to the superconducting cable 1 again is repeated. Patent Document 1 is an example of the cooling system which uses a method of cooling the coolant and supplying the coolant to the superconducting cable 1 in the above circulation cycle.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-12654
In the cooling system in which the coolant is cooled and supplied to the superconducting cable 1 in the above circulation cycle, it is necessary to control the temperature of the coolant at a coolant supply entrance to the superconducting cable 1 (the downstream side of the flow rate sensor 8 in FIG. 10) such that the temperature thereof has a constant value in order to maintain the cooling temperature of the superconducting cable 1 at a constant value. There are roughly the following three heat losses which occur when the superconducting cable 1 is cooled. They are (i) a loss caused by the amount of heat entering from the outside of the superconducting cable 1, (ii) a loss caused by an AC loss occurring when an AC current (or voltage) is passed through the superconducting cable 1, and (iii) a loss occurring in the circulation pump 5 which circulates the coolant. In particular, the heat loss by (ii) tends to change in response to a fluctuation in the load of the superconducting cable 1 during transmission of electric power.
In the above example, the control operation is performed by feedback-controlling the coolant temperature based on the detected values of the temperature sensor 7 and the flow rate sensor 8. Such a control operation is useful in a static system in which the heat loss is constant with time. However, there are cases where the heat loss occurring in the superconducting cable 1 fluctuates with time as described above and, in such cases, there is a problem that it becomes difficult to control the temperature of the coolant.
In addition, in the above example, the GM refrigerator or the Stirling refrigerator is used as the refrigerator 6. Accordingly, the control of the temperature of the coolant is performed by controlling an intermittent operation in the GM refrigerator or an operation cycle in the Stirling refrigerator. In a case where the refrigerator 6 of this type is used, in order to control the refrigerator 6, it is necessary to measure control parameters such as the temperature and the flow rate at a specific measurement point in the circulation path 9, and feedback-control the refrigerator 6. However, when consideration is given to the practical aspect of the superconducting cable, the scale of the superconducting cable is large with its length reaching several kilometers, and hence it is not easy to select the optimum position of the measurement point. In particular, while the length of the superconducting cable 1 is in the order of several kilometers, the actual expected velocity of flow of the coolant is assumed to be about a few tens of cm/s. As a result, a time period required for the coolant to make the circuit of the circulation path 9 reaches several hours, and a time constant of circulation of the coolant is extremely increased. On the other hand, the time constant related to the operation of the refrigerator 6 which performs the temperature control of the coolant (e.g., the clock frequency of a controller or the like) is extremely small, and is significantly different from the time constant of circulation of the coolant. Consequently, in the above example, in a dynamic system such as the case where the heat loss in the superconducting cable 1 fluctuates with time, there is a problem that it is not easy to control the entrance temperature of the coolant such that the entrance temperature thereof has a constant value.